This invention relates to electrochromic devices, especially electrochromic mirrors. More specifically, the invention relates to an improved electrochromic mirror which is characterized by its resistance to environmental and mechanical damage and to the making of such improved mirrors.
Electrochromic devices per se are known. Typically, such devices comprise thin layers of inorganic oxides that change color upon the application of an electric field, maintain the changed condition after the field is switched off, and return to the original state after pole reversal. Examples of electrochromic materials are tungsten trioxide and molybdenum trioxide, which are colorless and transparent when applied as thin layers onto a glass substrate or carrier. When a suitable voltage is applied to the layer, cations migrate from one side and electrons migrate from the other side to form the colored tungsten or molybdenum bronzes, as the case may be. The color intensity is a function of the amount of applied charge.
Electrochromic devices have been proposed for a variety of applications, including electro-optic displays such as electronic watches and for intentionally darkening optical glasses or mirrors. Electrochromic mirrors are described, for example, in U.S. Pat. Nos. 3,712,710; 3,844,636 and 4,465,339.
Electrochromic mirrors typically comprise a transparent conductive coating deposited on a piece of glass, followed by the sequential deposition of a number of active layers, and finally a conductive layer of aluminum, stainless steel or silver which can also function as a reflective surface. The active layers and aluminum, if used as a reflective surface, are sensitive to moisture and oxygen, with the efficiency and aging characteristics of the electrochromic cell being particularly responsive to water content. In addition, the various layers which make up the cell are sufficiently soft and fragile that they can be readily scratched or otherwise mechanically damaged, thereby negatively affecting optical properties of the mirror. When used as mirrors in the interior of automobiles, electrochromic mirrors have to be protected from impact. The shattered pieces of glass should be contained in the mirror casing to avoid injury to the occupants of the vehicle, either from shards of glass or leakage of corrosive materials from the electrochromic cells, some of which may contain acidic or basic liquid electrolytes.
Japanese Application No. 58-91431(A) discloses a solid state electrochromic display cell, comprising a series of thin films deposited onto a glass substrate which is sealed to protect the device from the ambient environment by covering the exposed surfaces of the thin films with a xylene polymer, which is itself covered with a reaction curing-type resin such as an epoxy resin to which a solid protecting plate can be adhered.
Baucke et al U.S. Pat. No. 4,465,339 discloses hermetically sealed electrochromic mirrors using an adhesive, such as an epoxide adhesive, applied over the stack of thin films deposited onto a transparent glass substrate, preferably in combination with a solid glass, plastic or metal backing plate adhered to the adhesive film.
Ganguillet et al U.S. Pat. No. 4,392,720; Bissar et al U.S. Pat. No. 4,227,779 and Amano U.S. Pat. No. 4,403,831 also disclose the use of adhesive resin or solder joints to seal the working elements of electrochromic display devices from the ambient environment.
While such techniques can be effective in hermetically sealing electrochromic devices from environmental factors, such as fluctuations in humidity, they are much less effective in protecting such devices from mechanical factors, such as impact loadings, because of the hard, brittle character typically exhibited by hermetic seal materials. In addition, the hard coatings can introduce stresses in thin films when applied directly onto the films.